I'm not an expert on the subject, but you can kind of force a sync by using the search option.

I just put the url from a part over there https://diyrpg.org/post/246 in the search box in our insurance and it started showing up here (without any of the comments)
Presumably, older comments could be synced manually with a link too, but that sounds like a lot of work and I haven't tried it.
If there a specific comment you wanna interact with is not too bad, don't this manually for all comments doesn't sound fun.
I think there might be browser extensions to "open this in your preferred instance" but I haven't tried any.

Silver lining is that stuff that happens after you first added the community should be more seamless.

yeah, the above is likely the issue. If there's a community with 0 subscribers on on this instance, then none of the content will sync. As soon as there is at least one subscriber here, all new stuff will begin to sync, but it will never sync the history over unless you explicitly search it out as mentioned below. This is how Lemmy is designed to work, and i assume the reason is because syncing a huge back-catalogue could be very problematic from a resources perspective.